Method and apparatus for controlling gas filtering devices



March 1, 1966 E. SCHWARZ ETAL 3,237,375

METHOD AND APPARATUS FOR CONTROLLING GAS FILTERING DEVICES Filed Dec. 16, 1960 4 Sheets-Sheet 1 .4 Lia/e NEAL? M31611 1, 1966 WARZ TAL 3,237,375

METHOD AND APPARATUS FOR CONTROLLING GAS FILTERING DEVICES Filed Dec. 16, 1960 4 Sheets-Sheet 2 641L117, M v- A t? arrays March 1, 1966 sc w z ETAL 3,237,375

METHOD AND APPARATUS FOR CONTROLLING GAS FILTERING DEVICES Filed Dec. 16, 1960 4 Sheets-Sheet 5 5 2 z l'W Inventors 2 @12 W mag:

March 1, 1966 E. SCHWARZ ETAL 3,237,375

METHOD AND APPARATUS FOR CONTROLLING GAS FILTERING DEVICES Filed Dec. 16, 1960 4 Sheets-Sheet 4 f Jn venfor:

United States Patent l 3,237,375 METHOD AND APPARATUS FOR CONTROLLING GAS FILTERING DEVICES Ernst Schwarz and Rolf Schlitt, Frankfurt am Main, Jean Wiemer, Oberhochstadt, Taunus, and Gunter Kopp, Dietzenbach-Steinberg, Germany, assignors to Metallgesellschaft Aktiengesellschaft, Frankfurt am Main, Germany Filed Dec. 16, 1960, Ser. No. 76,392 Claims priority, application Germany, Sept. 5, 1959, M 42,648; Dec. 23, 1959, M 43,806 11 Claims. (Cl. 55-4) This application is a continuation-in-part of our copending application Serial No. 54,074, filed September 6, 1960, now abandoned.

The invention relates to the cleaning of gases, and more particularly to the control of the quantity of dust contained in the cleaned gases. It involves both a method and an apparatus for controlling the process.

It is known that the efficiency of operation of an electrofilter, for instance one for cleaning blast furnace gases, can be very accurately followed by means of a control flame. The burnable gas is taken from the clean gas conducting means directly behind the cleaner and is dried and led to a gas burner. The image of the flame of the burned cleaned gas changes, as has been found, with the increasing dust content from a completely invisible blue (dust content below 1 mg./Nm. to a glowing red (dust content above 1 mg./Nm. (Nm. cubic meters at normal temperature.) Between these lie different graduations such as blue, milky bluewhite, rose spits, rose flame and the like. Only with very long experience is it possible to determine the dust content with accuracy by Watching the flame for changes in color. Even then subjective errors of different observers cannot be avoided, so that exact information about the dust content is not obtained.

It has heretofore been proposed to use an objective measuring procedure. In the past, blocking layer photo elements have been used for flame control. As tests show, however, these do not give satisfactory accuracy under certain conditions, especially in the most interesting range of between 1 and 20 mg. of dust per normal cubic meter (mg. Nm. if the photo element and control flame are assembled together in a simple fashion, because for example pressure changes in the cleaned gas, which effect the burner so that the height and strength of the flame change, change also the strength of observation of the photo element.

The primary object of the present invention is to provide a method and apparatus of measuring dust content which is more satisfactory than those heretofore known.

According to the invention, a process is provided for determining the dust content in the cleaned gases coming from a mechanical or electrical filter by using several filter installations and by providing an automatic input control for the individual filter installations, the known flame-photometric measuring processes being employed with the individual filter installations. The measurements received from a photocell independence on the flame color are converted into electrical energy, a separate threshold disturbing value switching arrangement is provided for each filtering arrangement, and through the disturbing impulse an input control regulation of the individual filter elements and consequently a maintenance of a maximum total dust content is achieved.

According to the invention a suitable gas burner, for example, a lean gas burner with baflles, is arranged in front of an observing tube. Only a part of the flame image is imposed by means of a slit diaphragm and a collector lens, which are arranged in the observing tube,

3,237,375 Patented Mar. 1, 1966 on the light sensitive element, which is likewise inside the observing tube. As a photocell, the invention preferably does not use a blocking layer photo element, but for example a cadmium sulphide photo resistance or other indicator, as for example a gas filled photocell with a caesium cathode or a photo conductor (lead sulphide photo conductor). This detecting element is arranged in a bridge circuit. Likewise, indicator elements can be used of the type which respond to a predetermined light frequency. The sensitivity of a bridge circuit, if a photocell is used in the circuit, can be changed as desired. Instead of the photocell, a photo resistance and photo conductor can be used, whose operation in the manner of switch elements is taken advantage of. In order to avoid the influence of outside light sources, the control flame is positioned in a light tight casing, which has a receiving support for the observing tube.

By using the photocell or photo conductor, it is not necessary to use a bridge circuit, but the electrical energy fluctuations, which are produced by the changes in color inside the indicator, can be supplied to different connections, and for example can be used as regulating impulses for the automatic volume control of the input of the filter. The electrical circuits used naturally conform to the mode of operation of the individual indicators.

For indicating and registering a recorder is used which consists of a zero-galvanometer in a bridge circuit. This apparatus is provided with a pneumatic amplifier which is sufficiently slow in its response so that sudden changes in opacity of short duration are not responded to, which is necessary in order to avoid registering short light flashes which result from the subsidence in the burner which under certain circumstances can be produced by vibrations, in which case the flame becomes glowing red for a short time.

Variations of the strength of the light produced by pressure changes are avoided by pressure reducing valves. It is especially important to carry out an adequate dehydration of the clean gas, which is usually super-saturated, because the pressure reducing valves and the burner nozzles are especially sensitive to water. If necessary, the conducting pipe is cooled in front of the dehydrating device and thereafter the gas is further dehydrated for the purpose of drying. The dehydrating can be carried out in any suitable dehydrating arrangement.

There are further possibilities for the use of flame photometers if several filters are in operation. For example, several photometers can be used with parallel connected filters for an automatic input control regulation of the individual filters, so that an optimal cleaned gas dust content is guaranteed at the output of the filter arrangements, which is especially advantageous in filters operating by rinsing.

If, for example, at the beginning of operation the filters in a known way are adjusted to equal input quantities, it is not normally possible, because of the variable dust precipitation and the variable degree of removal of dust connected therewith, to maintain the total dust removal constant. But if a threshold value disturbing circuit is used with the aid of flame photometric measurement, an input control, for example, automatic, can be obtained and, through the control at the same flame color of all the filters, a constant total dust removal efliciency can be obtained although the different filter elements are fed with variable amounts of raw gas. Also unimportant flame colors can be eliminated, in order, for example, to put the cadmium sulphide photo resistance in operation as a Warning device.

By the so-called threshold value disturbing circuit with the help of the flame measurement the individual gas quantities of several individual filters are regulated in dependence on the divergence from a predetermined theoretical value of the dust content in the cleaned gas of each.

In order to prevent a too great loss of energy by backpressure on a pump or the like, which might be produced by too strong a closing of the valve elements used for regulating the quantity of gas in the gas inlet to the filter, according to another advantage of the invention, the selected theoretical value of the photometric regulating arrangement is made dependent on the differential pressures between the inlets and outlets of the filters, the theoretical value being changed with increasing pressure difierence in the direction of a greater content of dust in the cleaned gas. This produces an opening of the valve elements if the total flow drops too low, despite the presence of an excessive amount of dust in the gases leaving the filters.

According to a further feature of the invention, gases which are not themselves combustible can be measured as to gas content with the same efiect, by adding a stream of the gas to be tested in a flame fed with a different combustible gas, so that a coloring of the flame is produced by the burning or at least the glowing of the dust and haze contained in the gas. This procedure can be carried out by flame photometric measurement.

In a variation of the process of the invention, a dust content measurement with a threshold disturbing value circuit can be carried out on incombustible dust-containing gases also, by using a known light detector, which for example, works with a selenium cell, to measure the turbidity. The resulting measured values are then used in a known way for regulation. In this case then there is not a photometric measurement but a turbidity measurement.

The advantage of the process is to make possible a high accuracy of measurement of dust content in incombustible gases.

The process according to the invention and the apparatus for carrying it out present, over the conventional methods, a substantial improvement, because of the simplicity of supervising, servicing and automatically controlling the filter arrangement which results from the high accuracy of the measurements. In apparatus which is working poorly or unsatisfactorily, the photometric dust measuring process gives immediate measurement and recording of information concerning the operation, for example, in testing with different spray nozzles, water pressure or changes in the gas generator.

Whereas in a cleaned gas dust measurement, because of the time heretofore necessary for measuring and evaluating, only a mean value over the whole time of measurement could be obtained, the operation of electrofilters with different currents in the individualfields in the cleaning operation can very quickly be determined. By taking off the measurements of the arrangement, the photometer can serve well as a control apparatus and the recorded time diagram can thereafter be used to ascertain small errors.

Further objects and advantages of the invention will appear more fully from the following description especially when taken in conjunction with the accompanying drawings, which form a part thereof.

In the drawings:

FIG. 1 shows schematically an apparatus for carrying out the invention;

FIG. 2 is a circuit diagram of a part of FIG. 1;

FIGS. 3 and 4 show graphs for the galvanometer or recorder in the bridge circuit which shows percentages in dependence on the gas content;

FIG. 5 shows schematically a plurality of filters with an automatic input regulation of the individual filters;

FIG. 6 is a further explanatory diagram;

FIG. 7 shows a control circuit for a filter; and

FIG. 8 shows a burner for testing incombustible gases.

In the apparatus shown in FIG. 1, the cleaned gas pipe is shown at 1. Through a valve 2 and a pipe 3 the gas is led to a dehydrating arrangement 3'.

From this it passes through the pipe 4 and the pressure reducing pipe 5 through a further valve 6 into the lean gas burner 7, which is provided with baflies 13. The lean gas burner is arranged in a light tight control flame casing 8, in which an observation tube 9 with a slit diaphragm and a condenser lens 11 as well as the cadmium sulphide photo resistance 12 are arranged. The cadmium sulphide photo resistance is connected in the measuring bridge 15 by the connections 14 and 14'. The zero galvanometer 20 is connected by the connections 17 and 18 in the bridge. Instead of this galvanometer, a recorder could be used, the current supply of which is indicated at 20. As balancing resistances in the bridge circuit are provided the resistance 28a (constant) and 28 (variable) with a midtap 17'. The current supply of the measuring bridge is connected with it through a known type of net arrangement, which connects it to a transformer 21 with a secondary additional winding 22. This supply net includes a rectifier 23, and circuit elements for smoothing the rectified current, such as an ohmic resistance 24, a condenser 25 and a glow stabilizer 26. There is also provided a variable resistance 27 for the regulation of the sensitivity of the circuit. The supply net is connected to the bridge at the points 16.

At the same time that the photocell responses are registered by the recorder, normal cleaned gas withdrawals are carried out. The cleaned gas dust content determined thereby (points marked with crosses in FIGS. 3 and 4 in which the abscissas represent dust content in milligrams per normal cubic meter, while the ordinates represent efiiciency of dust removal in percentage) is compared with the planimetric mean value of the photographic indicators shown by the broken curve. This gives in FIG. 3 six measuring points, which lie almost exactly on the curve. In FIG. 4 the values of FIG. 3 are set up logarithmically, so that the corresponding straight line can be used as a reference curve, with the help of which the scale of the indicator apparatus can be read directly in mg./ Nm.

If some other indicator than a photocell is used, for example, a photo conductor or a photo resistance, and a quantity regulating arrangement for the automatic input control for the automatic input regulation of several filter units, an arrangement such as that of FIG. 5 results.

The raw gas canal is shown at 30, from which pipes 30a, 30b, 30c, 30d lead to the inlets of filter units 31a, 31b, 31c, 31d. The cleaned gas outlet pipes 32a, 32b, 32c, 32d open into the cleaned gas outlet 32. From the pipes 32a to 32d, pipes 33a to 33d lead to burners 34a to 3401'. The dehyd'rators and pressure reducing valves are indicated by 35a to 35d. Indicators 36a to 36d detect the color changes of the flame in the burners and convert these color changes into electrical impulses which are led through switch members 37a to 37d to a conventional automatic measuring arrangement 38. From this, the automatic regulation of the gas volume for the input control of the filter arrangment to maintain the gas volume constant is 'such that, if Q is the total gas volume, and q to g the partial gas volumes in the individual raw gas tubes 30a to 30d, then q +q +q +q =Q with the condition that the total change of volume is always zero, or q1+ q2+ q3+ q4= In order to accomplish this, a theoretical value of the cleaned gas dust content is furnished to the regulator 38, by which the input of the filters 31a to 31d is automatically controlled.

In order to avoid too high a loss of energy by back-pressure against a pump or the like, because of the temporary occurrence of high dust content in the raw gas and the consequent closing of the valve elements of the filters, the theoretical value of the regulator is made dependent on the pressure difference between the inlets and outlets of the filters, so that, when a predetermined pressure difference is reached, the theoretical value is changed in the direction of higher dust content in the cleaned gas, so as to permit the flow to continue substantially constant, although the outgoing gas will have an excessively high dust content. The eight differential pressure measuring devices are shown at 39a to 39/1.

The diagram (FIG. 6) gives an example of the regulation over a period of time. In this diagram, there are shown, with reference to time, the curves for the cleaned gas dust content p, the differential pressure AP and the gas volume Q. If the rate of separation in one of the filters 31a to 31d decreases, at a time T the regulator responds after a certain time AT=T T This causes the valve elements 42 of the filter whose operation has deteriorated to close further. The differential pressure AP, beLween the pressures behind and in front of the filter then increases. The throughput of gas is likewise smaller. This means that, up to a time T the pressure difference increases to a certain value, the cleaned gas dust content is however always maintained at a determined theoretical value 'l'h. After time T the proportions remain constant.

If the raw gas volume increases to time T because of a disturbance in the furnace or the like, this produces a further increase in the differential pressure, because the valve setting is not changed. If the differential pressure at time T reaches its preselected maximum AP the theoretical value of the cleaned gas dust content is changed upwardly to 'Th. Under the given conditions until at time T the cleaned gas dust content, for example through reduction of the total gas content, has returned to its original value Th, the ratios for the differential pressure and gas volume remain constant. If the theoretical value drops below the value Th at time T the differential pressure decreases, which means that the valve opens further and the gas volume increases. For clarity, the gas volume measuring devices are not shown in FIG. 1.

The input volume control is carried out by the controller 38, through relays 40a to 40d, which operate regulators (motors, magnetic controls or the like) 41a to 41d in raw gas pipes 30a to 30d, so that the throughput Q of gas remains constant and the sum of all the partial volume changes dq is zero.

FIG. 7 shows a control arrangement for use in the circuit of FIG. 5. For each filter a similar control arrangement is provided.

Assume that color frequency of the flame of the burner 34 of the indicator 36a, which is connected in the bridge circuit, is to be measured. If the color of the flame varies from a predetermined value which is preset on the resistance 28, the instrument 29, on the shaft of which a transmitter 37 is positioned, moves. The transmitter 37 then produces, in dependence upon the deviation of the color from the predetermined value, a current the phase and amplitude of which is dependent on the direction and the strength of the current in the crosspiece of the bridge, and this current is imposed on the voltage divider (variable resistance) 44 arranged in the regulating arrangement. This voltage divider lies in a compensation circuit with the voltage dividers 43 and 48, which also includes the primary winding of the input transformer of the amplifier 45. If the vectoral sum of all the currents is equal to zero, there will be no current induced in the input transformer. Because no other current opposes the current in the voltage divider 44, this will be amplified and a contact switch 46 will be moved in dependence on the phase to switch on one of the other of the reversing switches 40, which puts the control motor 41 in operation.

Since the control motor is connected to the pressure reducing valve 41, the quantity of gas which flows through the filter will then be changed. Simultaneously the transmitter 49, which is connected with the shaft of the motor 41, produces a countercurrent the value of which is imposed on the voltage dividers 47, 48. The control arrangement then comes to rest when the flame in the burner 34 shows the color whose actual value corresponds to the theoretical value set on the control arrangement 38. If now the pressure drop across the filter becomes so great as to exceed the value to which the differential measuring device 39 is set, this causes switching on of switch 42a which in turn causes the turning of transmitter 42 which supplies a current through the voltage divider 43 to the compensating circuit, which opposes the current from the voltage divider 44. If the sum of the currents is opposed in phase to the current at the voltage divider 44, then through the amplifier 45 the contact 46 and reversing switch 40 of the control motor are closed to operate the control motor in the direction of opening of the pressure reducing valve. If the pressure falls further or burner 34 further shows a gas of better cleanliness, the regulating arrangement comes again into operation, however with an inverse phase, so that it operates to produce a smaller pressure drop.

For incombustible gases, an arrangement such as shown in FIG. 8 can be used. In this case, only one burner head 34 is shown, this being representative of all the heads 34a to 34d. The tube 33, corresponding to tubes 33a to 33d, brings an incombustible gas to the burning head. It is enclosed by second burner tube 51, which is fed through the opening 52 with combustible gas. This gas goes out through the opening 53, where it is burned. In front of the outlet opening 54 of the burner tube 33 the incombustible gas is blown into the flame.

This arrangement is used in connection with the drying and pressure reducing arrangements shown in FIG. 5.

The threshold disturbing value switching arrangement is described in E. P. Popow, Dynamik automatischer Regelsystem, Akadamie-Verlag, Berlin, 1958, page 13. Control of the disturbing value means an automatic control, in which the regulating value is not measured and the opposing of the disturbing influence is produced through the interposition of arrangements which do away wih the operation of this influence.

While we have described herein some embodiments of our invention, we wish it to be understood that we do not intend to limit ourselves thereby except within the scope of the claims hereto or hereinafter appended.

We claim:

1. Process for operating an electrofiltering mechanism which comprises passing gas through a plurality of electrofilters in parallel, determining the dust content of the gas issuing from each electrofilter, and cont-rolling the input of gas to each of the electrofilters so as to maintain the flows through the different electrofilters substantially inversely proporional to the relative percentages of dust content of the gas leaving each electrofilter.

2. In a process as claimed in claim 1, maintaining the total flow of gas through all the electrofilters substantially constant.

3. In a process as claimed in claim 1, the step of increasing the dust content of the gas issuing from the mechanism in response to increase in the pressure differential across at least one of the electrofilters above a predetermined value.

4. A process as claimed in claim 1 in which the gas issuing from the electrofilters is dried and thereafter burned and the dust content determined by color changes in the flame.

5. A process as claimed in claim 4 in which the pressure of the gas to be burner is kept constant.

6. Electrofiltering apparatus for cleaning gas comprising a plurality of electrofilters, means to pass gas through the electrofilters in parallel, means to determine the dust content of the gas issuing from each electrofilter, and control means responsive to such determination to maintain the flows through each of said electrofilters substantially inversely proportional to the relative percentages of dust content of the gas leaving each electrofilter.

7. In apparatus as claimed in claim 6, said dust content determining means comprising photometric means.

8. In apparatus as claimed in claim 6, said dust content determining means comprising burners, means to supply gas from the exit of each electrofilter unit to one of said burners, a tube containing a light-sensitive device and a slit diaphragm and condenser lens between the burner and the light-sensitive device.

9. In apparatus as claimed in claim 8, a light tight casing surrounding said burner and having means for holding said tube in a position exposed to the burner.

10. In apparatus as claimed in claim 8, having pipes to conduct gas from the outlet of each eleetrofilter to one of the burners, a drier and a pressure-reducing valve in each of said pipes.

11. In apparatus as claimed in claim 6, in which said control means has an element settable for a predetermined value of the dust content, means responsive to an increase of the diiference in pressure between the inlets and outlets ment to increase the dust content above said predetermined value.

References Cited by the Examiner UNITED STATES PATENTS 1,337,488 4/1920 Strong 55133 1,379,056 5/1921 Smith '5521 2,667,779 2/1954 Von Brand 73--28 2,730,005 1/1956 Vonnegut 7328 X 2,819,774 1/1958 Schmidt et al 55270 X 2,949,976 8/1960 Tuthill 55274 REUBEN FRIEDMAN, Primary Examiner.

of the filter units above a selected value to set said ele- 15 WESLEY COLE: Examiner- 

1. PROCESS FOR OPERATING AN ELECTROFILTERING MECHANISM WHICH COMPRISES PASSING GAS THROUGH A PLURALITY OF ELECTROFILTERS IN PARALLEL, DETERMINING THE DUST CONTENT OF THE GAS ISSUING FROM EACH ELECTROFILTER, AND CONTROLLING THE INPUT OF GAS TO EACH OF THE ELECTROFILTERS SO AS TO MAINTAIN THE FLOWS THROUGH THE DIFFERENT ELECTROFILTERS SUBSTANTIALLY INVERSELY PROPORTIONAL TO THE RELATIVE PERCENTAGES OF DUST CONTENT OF THE GAS LEAVING EACH ELECTROFILTER. 